The present invention relates to a supercharged two-stroke or four-stroke internal combustion engine having one or more cylinders, and operating by admitting a carburated mixture or by admitting fresh air with the direct or indirect injection of fuel. The invention is just as applicable to petrol engines equipped with spark plugs as it is to diesel engines which use compression ignition.
Although the invention is described hereinafter with more particular reference to a single-cylinder engine in the case of a two-stroke engine, which is well suited to all applications of small industrial engines intended for motorized cultivation, garden tools, lawn mowers, cutters, scrub clearers or the like, the invention is not in any way restricted thereto and is also applicable to two-stroke or four-stroke multi-cylinder in-line or V engines.
A two-stroke single-cylinder engine which operates with natural aspiration into the cylinder of a carburated mixture which passes through the crankcase is already known. This engine has a pipe for admitting the air/fuel mixture and a pipe for exhausting the burnt gases, both of which pipes open in the form of ports toward the bottom of the cylinder, near bottom dead center (PMB). The carburated mixture from the carburetor is drawn into the crankcase through a valve, during the upstroke of the piston which causes a depression in the crankcase, and is then delivered to the cylinder, during the downstroke of the piston, causing a raised pressure in the crankcase. During the downstroke of the piston, the mixture inlet ports are open at practically the same time as the exhaust ports, which means that about 20% of the mixture is discharged directly to the exhaust, leading to a high fuel consumption and a great deal of atmospheric pollution. The main advantage of this engine is its low cost, but new antipollution standards will ultimately spell the end for this type of engine.
Another known engine is of the loop scavenging type, which operates with a positive-displacement compressor, for example of the Roots type, making it easier to introduce the carburated mixture into the cylinder and to generate low-pressure supercharging. This engine also has a mixture inlet pipe and an exhaust pipe, the pipes both opening via ports toward the bottom of the cylinder. In this engine, the carburated mixture is admitted into the cylinder from the compressor, with an orientation such that the mixture experiences a loop-like upward rotating movement after the manner of a xe2x80x9cloop-the-loopxe2x80x9d in the cylinder, while the burnt gases from the previous cycle are discharged to the exhaust ports. The particular arrangement of the inlet and exhaust ports makes it possible for part of the admitted mixture not to be exhausted directly, and this reduces both fuel consumption and environmental pollution.
Yet another known engine is of the uniflow type, which also operates using a positive-displacement compressor. This engine has an inlet pipe connected at its upstream end to the compressor and at its downstream end to an inlet ring which opens via a number of ports toward the bottom of the cylinder, with an orientation such that the mixture is introduced with a great deal of rotational movement. The burnt gases are discharged at the top of the cylinder through one or more exhaust valves. This type of engine allows control over the filling of the cylinder and the possible recirculation of burnt gases, so as to obtain combustion which causes less pollution. Furthermore, when this type of engine is operating on the diesel cycle, introducing the air near the bottom of the cylinder makes it possible to obtain a great deal of air rotation, which is needed for obtaining good efficiency. This engine makes it possible to consume even less fuel than the loop-scavenging engine, and also makes it possible to reduce polluting emissions.
However, these last two types of engine cost far more than engines with transfer via the crankcase, because they contain more parts, particularly the compressor, and furthermore, in the case of the uniflow engine, valve control means. Furthermore, compressors of the Roots type are of low efficiency; for example, a two-stroke single-cylinder engine with a one-liter cylinder capacity and a power of 55 kW will consume 17 kW for driving the compressor. What is more, a Roots compressor does not operate beyond a pressure higher than 1.2 bar.
Finally, engines with exhaust and inlet valves are known and these are able to obtain the lowest consumptions and the lowest emissions, but this type of engine is also the most expensive because both the exhaust valves and the inlet valves have to be controlled. The efficiency of this engine is better because the control of the opening and closing of the valves using parts external to the cylinder means that the entire piston stroke can be used whereas with the previous engines in which admission was via ports, part of the compression stroke and of the expansion stroke was lost.
The object of the invention is to provide a supercharged two-stroke or four-stroke internal combustion engine, for example of the loop scavenging, uniflow or valve type, or of the four-stroke valves type, which allows the efficiency to be improved and the emissions to be reduced.
To this end, the subject of the invention is a two-stroke or four-stroke internal combustion engine, operating by admitting a carburated mixture or by admitting fresh air with the direct or indirect injection of fuel, the engine having at least one cylinder defining a variable-volume combustion chamber in which an engine piston coupled by a connecting rod to the wrist pin of a crankshaft executes a reciprocating movement, and a compressor associated with each cylinder in order to supercharge the cylinder with carburated mixture or with fresh air, characterized in that said compressor is a compressor with at least one stage, in the compression chamber of which there moves a compressor piston which is coupled to the crankshaft by a link rod articulated to an eccentric, said eccentric being mounted on the shaft of said crankshaft.
As a preference, the angle of the dihedron, the solid angle of intersection of which is formed by the axis of the crankshaft and the two half-planes of which extend one toward the eccentric and the other toward the wrist pin, is of the order of 90xc2x0 so as to obtain a phase shift between the top dead center (PMH) positions of the engine piston and of the compressor piston which are associated with the same cylinder, which phase shift ensures that the pressure in the compression chamber is at its maximum before the carburated mixture or the fresh air is admitted into the combustion chamber.
In this case, when the stage of the compression chamber which communicates directly with the cylinder is located between the compressor piston and the crankshaft, the wrist pin has a phase shift in advance of the eccentric in the direction of rotation of the crankshaft and, conversely, when the aforementioned stage is on the opposite side of the compressor piston to the crankshaft, the eccentric has a phase shift in advance of the wrist pin in the direction of rotation of the crankshaft.
Advantageously, the cylinder capacity of the compressor is of the order of magnitude of that of the cylinder, but with a compressor piston which has a diameter markedly greater than the diameter of the engine piston, so that the compressor piston has a short compression stroke in the compression chamber.
In a particular embodiment, the compressor piston is rigidly attached at its center to the link rod for connection with the eccentric so that the compressor piston moves in the compression chamber by rocking back and forth about lower and upper parts of the compression chamber, the axis of the compressor being offset, in the direction of the axis of the crankshaft, with respect to the axis of the cylinder. In this case, the compressor piston can have, at its periphery, a spherical edging fitted with a spherical sealing ring which is preferably unable to rotate with respect to the compressor piston, in a position such that the gap in the ring is not placed at the bottom of the compressor, so as to limit the oil consumption and therefore the environmental pollution.
In another embodiment, the compressor piston is secured at its center to a rod articulated to the link rod for connection to the eccentric, said rod being guided in translation in a direction which intersects the axis of the cylinder. In a first alternative form, the compressor piston is a deformable diaphragm connected at its periphery to the side wall of the compression chamber, said diaphragm preferably having an undulation at its periphery, to make it easier to deform. In a second alternative form, the compressor piston is a rigid cylinder which can move in axial translation and is fitted at its periphery with at least one sealing ring.
This second embodiment is advantageous in that it carries no risk of oil passing between the crankcase and the compression chamber of the compressor, because it is possible to arrange a seal or a sealing boot on the compressor piston rod.
In one particular embodiment, the compression chamber has two stages located one on each side of the compressor piston, a first stage being supplied with carburated mixture or with fresh air by a first nonreturn valve or a valve, and connected by a delivery duct fitted with a second nonreturn valve or a valve to the second stage which communicates with the cylinder via an inlet duct possibly fitted with a third nonreturn valve or a valve. The use of a two-stage compressor makes it possible to obtain a higher boost pressure in the cylinder. However, in this case, the volumetric ratio of the cylinder may be reduced so as not to reach a maximum combustion pressure which is incompatible with the mechanical strength of the cylinder. The engine equipped with this two-stage compressor will work in a similar way to the known hyperbaric-type supercharging system.
The two-stroke engine of the invention may also be fitted with a device for recovering the energy in the exhaust puffs and for partially recirculating the exhaust gases by providing an additional volume communicating with the cylinder through closure and opening means, the movements of which are controlled either in synchronism or with a phase shift with respect to those of the engine piston in the cylinder so that during the expansion phase, the burnt gases compress the air in the additional volume and at least partially enter it, so that this air and burnt gases mixture is trapped under pressure therein, and then so that this mixture is admitted into the cylinder during the compression phase.
Advantageously, after the air and burnt gases mixture previously trapped in the additional volume has been admitted into the cylinder, said additional volume is once again filled with fresh air from the compressor.
According to another feature, the aforementioned closure and opening means comprise two rotary shutters, for example multi-way rotary spools, connected to each other by the additional volume, one of the shutters being associated with the compressor, and the other shutter being associated with the exhaust from the cylinder.
As a preference, the two rotary shutters are arranged in such a way that the following operations take place: in a first phase, when the engine piston is near its PMH, a flow of air from the compressor passes through the lower shutter associated with the compressor, sweeps through the additional volume, passes through the upper shutter associated with the exhaust and is exhausted to the outside via an exhaust manifold; in a second phase, from about halfway through the expansion stroke of the engine piston, on the one hand, the upper shutter places the cylinder in communication with the additional volume so as to fill it with a pressurized mixture of air and burnt gases and, on the other hand, the cylinder communicates with the exhaust; in a third phase, the upper shutter traps the air and burnt gases mixture in the additional volume; in a fourth phase, air from the compressor is admitted into the cylinder and, in a fifth phase, at the start of the engine piston compression stroke, the trapped and pressurized mixture is admitted into the cylinder.
In a first alternative form, the upper shutter is associated with at least one exhaust valve located at the top of the cylinder and the lower shutter is connected to the cylinder by a pipe arranged toward the bottom of the cylinder so that the additional volume is pressurized via its upper end by the burnt gases from the exhaust valve through the upper shutter and is emptied into the cylinder via its lower end through the lower shutter.
In a second alternative form, the upper shutter is connected to the cylinder by a pipe arranged toward the bottom of the cylinder and the lower shutter is fitted on the delivery pipe between the two stages of the compressor so that the additional volume is pressurized by means of the burnt gases from the cylinder through the upper shutter and is emptied into the cylinder through the pipe connected to the upper shutter.
Advantageously, in the case of two-stroke or four-stroke engines, the inlet pipe to the cylinder and/or the delivery pipe from the two-stage compressor is cooled by any appropriate means.
The two-stroke engine may be of the loop scavenging type, in which the carburated mixture or the fresh air is admitted from the compressor through an inlet duct opening via ports into the lower part of the cylinder with an orientation such that the mixture or the air is introduced with a looping upward rotating movement, while the burnt gases from the previous cycle are discharged through exhaust ports also arranged toward the bottom of the cylinder.
The two-stroke engine may alternatively be of the uniflow type, in which the carburated mixture or the air is admitted toward the bottom of the cylinder through inlet ports distributed at the base of the cylinder and supplied by a ring, itself connected to the compressor, while the burnt gases from the previous cycle are discharged through one or more exhaust valves located at the top of the cylinder.
Finally, the two-stroke or four-stroke engine may be of the type with exhaust and inlet valves, in which the valves are located at the top of the cylinder and the inlet valve or valves are supplied by the compressor.
The invention is also applicable to an engine of the type with several in-line cylinders, in which the compressors associated with each cylinder are arranged alternately on each face of the crankcase.
To allow better understanding of the subject matter of the invention, several embodiments thereof depicted in the appended drawing will now be described by way of purely illustrative and nonlimiting examples.